This invention is preferably used for relatively long pressure rollers, in which the dead load of the roller as well as the effect of the said contact surfaces result in general in an imperfect contact between the supporting surfaces and the pressure roller. It ensues therefrom that the contact pressure is not uniformly distributed along the contact line, but tends to vary from point to point of the line according to complicated laws reaching a minimum effect in the midst of the pressure roller. If it is necessary to raise this minimum to a given value, a pressure must be applied to the roller corresponding to a mean pressure value along the jacket line of the roller, rather higher than the value of the said minimum.